1. Field of the Invention
The present invention relates to variable dispersion compensators, and more particularly, to an optical dispersion compensator suitable for application to optical transmission systems using optical fibers, and systems using an optical transmission scheme based on wavelength division multiplexing.
2. Description of the Related Art
In resent years, research and development have been conducted on long-distance optical transmission systems that use optical amplifiers as repeaters. It is considered to be particularly effective to increase a capacity using wavelength division multiplexing (WDM) technique. The WDM technique multiplexes a plurality of optical signals with different wavelengths over a single optical fiber used for a communications transmission line in order to support Internet-based multimedia services. In the long-distance optical transmission systems, transmission speeds and transmission distances are significantly limited by the wavelength dispersion of the optical fiber. The wavelength dispersion is a phenomenon in which light beams with different wavelengths are propagated through the optical fiber at different speeds. Since the optical spectra of the optical signals which have been modulated at a high speed have different wavelength components, the components are affected by the wavelength dispersion occurring during the propagation through the optical fiber and may not reach a receiver simultaneously. As a result, a distortion may be generated in the waveform of an optical signal after the transmission through the fiber.
The technique of dispersion compensation is important to reduce waveform deterioration due to such dispersion. Dispersion compensation is a technique for canceling unnecessary characteristics of wavelength dispersion of an optical fiber and thus preventing waveform deterioration. This technique is realized by disposing, at an optical transmitter, receiver, repeater or the like, an optical element that has wavelength dispersion characteristics opposite to those of the optical fiber used for a transmission line. Research and development have been conducted on the devices having such opposite dispersion characteristics, such as dispersion compensating fibers and optical fiber gratings (e.g., refer to Japanese Patent Laid-open No. 10-221658) used as such an optical element, that is, a dispersion compensator.
Dispersion tolerance indicates the range of the residual dispersion which satisfies certain transmission quality standards (i.e., the total amount of optical dispersion due to the transmission line fibers and the dispersion compensators). Since dispersion tolerance decreases in inverse proportion to the second power of the bit rate of an optical signal, the dispersion compensation technique becomes more important as the transmission speed increases. For example, in a 10-Gbps transmission system, the dispersion tolerance of optical signals is about 1000 ps/nm. Thus, in consideration of the fact that the amount of dispersion of a single-mode fiber (SMF) is about 17 ps/nm/km, the system can transmit an optical signal only over a distance of about 60 km without using the dispersion compensation technique. The dispersion tolerance in a 40-Gbps transmission system is about 60 ps/nm, which is about 1/16 of that in the 10-Gbps transmission system. Thus, in an SMF, an optical signal can be transmitted only over a distance of about 4 km.
The transmission distances of currently used trunk optical fiber transmission lines with use of optical repeaters are from several tens of kilometers to several thousands of kilometers. The amount of dispersion compensation of the dispersion compensator used, however, needs to be changed according to the transmission distance. For a 10-Gbps transmission system, for example, a dispersion compensator having a prefixed amount of dispersion compensation has been prepared for each 100 ps/nm to several hundreds of ps/nm in consideration of the dispersion tolerance, and has been installed after determining the appropriate amount of dispersion compensation according to the transmission distance. In this case, the dispersion compensator typically uses a dispersion compensation fiber having wavelength dispersion of a sign opposite to that of the transmission line. Likewise, it is considered to be necessary for a 40-Gbps transmission system to install a dispersion compensator capable of changing an amount of dispersion compensation for each 5 ps/nm to several tens of ps/nm. The amount of dispersion compensation corresponds to a transmission distance of several hundred meters in the case of using a SMF. Thus, if a dispersion compensator with a fixed dispersion compensation is used, it is required to prepare a dispersion compensator based on the transmission distance. In addition, in this case, changes in the amount of wavelength dispersion due to changes in the temperature of the transmission line fiber cannot be ignored. It is therefore required to provide a dispersion compensator capable of variably controlling an amount of dispersion.